In worm conveyors for bulk material, which are designed to convey into a higher-pressure outlet chamber, it is known (EP-B 98 340, FR-A 755 192, DE-C 886 268) to provide at the outlet end of the worm conveyor a non-return flap. The non-return flap is pressed by weight, spring force or pneumatically into the closed setting. Without a non-return flap, the pressure differential between the outlet and the inlet of the worm conveyor would result in a gas flow in the worm conveyor counter to its direction of conveyance. The counter directional gas flow would render the reception of bulk material at the inlet end impossible. As soon as the worm conveyor is filled with bulk material which is suitable for plugging, the counter-directional gas flow can be sufficiently suppressed insofar as, at the inlet of the worm conveyor, a sufficient supply of new bulk material is guaranteed. Since this condition cannot however always be met and since there are also bulk materials which, due to their coarseness, grain spectrum or consistency, exhibit little tendency to plug, the flap function is generally necessary during operation also. The material leaving the conveyor raises the flap from its seat, whereupon the flap, due to being flexibly stressed by spring or weight, continues to exert a compressive and cohesive effect upon the material located in the outlet end of the conveyor and thereby contributes to the formation of a plug. This effect is all the more pronounced, the stronger the force by which the flap is forced into the closed setting. In the case of so-called "difficult" material, which exhibits little tendency to plug and in which there is a danger of counter-directional gas flow, there is not only impairment of the conveying effect but also possibly the reception of the material is inhibited. The flap-closing force is therefore set at a high level. This has the drawback, however, that the material encounters a high discharge resistance. A higher drive power is necessary for the conveyor worm, wear increases and the temperature of the conveyor material is possibly inadmissibly raised.
In the known machines, a brisk movement of the non-return flap is evident during operation. It was assumed that this was a necessary concomitant of the regular flap function, because it seemed necessary for the flap to be able to adapt itself without delay to the varying conditions of the material discharge at the worm outlet. The rapid response was required in order, on the one hand, to prevent the discharge of material from being unnecessarily inhibited and, on the other hand, to enable a sufficient retention effect to be exerted at all times upon the material being discharged.